Having done probably a dozen+ flights in UA intl. F over the last 24 months, I always got my first choice, so in my experience this seems not to be a major issue. From what I understand UA loads extra meals in intl. F, vs. C: quite often FAs come up to pick some food, left from the dinner service. I've seen pax in F being given a limited choice a couple of times, but they were SAs and seem to understand.

Now, domestic F is a different story, as those flights load no extras, except maybe premium transcons.